1. Field of the Invention
This invention relates to systems and methods for improving a power and bandwidth constrained transmission system, and in particular to a system and method for adaptively varying an uplink signal from a terrestrially-based user terminal to a satellite according to a predicted degradation of the uplink signal.
2. Description of the Related Art
A typical communications system, whether terrestrial or satellite-based, is constrained in its transmission power and signal bandwidth. The communications system may also be required to deliver reliable communications to one point, as in a terrestrial microwave link, or to many points in a satellite broadcast system serving the United States.
A typical satellite communications system is typically designed such that all signals transmitted to and received from terrestrially based receivers have a minimum carrier to noise ratio (CNR). The CNR of the received signal is a function of a number of parameters, including the power of the signal transmitted and signal propagation characteristics such as atmospheric attenuation. Unfortunately, at the wavelengths typically used in satellite-home communications systems, rain and other sources of atmospheric attenuation have a significant effect upon the CNR. Hence, during heavy rain, subscribers can experience degraded signal quality in both transmitted and received signals.
In the past, system designers have assured a minimum signal quality is provided by designing transmitters, power systems, and related components to produce sufficient power to assure a minimum CNR in all conditions, even when compromised by unfavorable atmospheric attenuation characteristics. Unfortunately, the amount of attenuation from rain and similar atmospheric parameters can be substantial and widely varying. Hence, to achieve the desired CNR at all times (including intense rain periods) this solution requires that the ground and satellite transmitters, power system and related components be designed for high performance levels that are not needed most of the time.
Setting the transmitter power to a level that results in a sufficient CNR for all conditions (or nearly all conditions) may be economically impractical, and is typically infeasible in existing field units. What is needed is a system and method for adaptively controlling transmissions to account for signal propagation characteristics. The present invention satisfies that need.